1. Field of the Invention
The present invention relates to an image forming method in which a latent image is formed on a photosensitive member which contains an organic photoconductive material (hereinafter "OPC photosensitive member") and serves as a latent image bearing member, said latent image is developed with a toner, and the developed image is transferred to a transfer medium to obtain a visible image, and then the latent image bearing member is cleaned after the transfer by means of a cleaning member for repeating use. It also relates to a toner for developing an electrostatic image, suited to such a method.
2. Related Background Art
In recent years, various electrophotographic image forming apparatus such as color copying machines and printers making use of OPC photosensitive members as a latent-image bearing member are produced. In such an image forming apparatus, usually a latent image is formed on an OPC photosensitive member by a usual electrophotographic process, the latent image is developed by a developing assembly to form a developed (toner) image, end the developed image is transferred to a transfer medium to obtain e visible image. Meanwhile, the residual developed image on the latent-image bearing member is cleaned by means of a cleaning member, end the latent-image bearing member is repeatedly used.
As the cleaning means, a blade-cleaning means comprising pressing a cleaning blade made of an elastic rubber material to the latent-image bearing member, is very popular because of its simple and compact construction and cost effectiveness.
In general, an OPC photosensitive member comprises a conductive support end, provided thereon in the following order, a charge generation layer comprising e binder end dispersed charge-generating material and a charge transport layer comprising a binder end dispersed charge-transporting material. As a charge-generating material, pigments such as phthalocyanine pigments, enthrone pigments, azo pigments and indigo pigments or dyes such as cyanine dyes are used. As the charge-transporting material, carbazoles such as pyrene and isopropylcarbazole, hydrazones, pyrazolines, oxazonyl compounds, thiazole compounds, triarylmethane compounds and polyarylalkane compounds are used. As a binder, used are polyacrylate resins, polystyrene resins, polyamide resins, acrylic resins, acrylonitrile resins, methacrylic resins, vinyl chloride resins, vinyl acetate resins, phenol resins, epoxy resins, polyester resins, alkyd resins, polycarbonates, polyurethanes, or copolymer resins containing at least two of any of these resins as repeating units as exemplified by a styrene/butadiene copolymer, a styrene/acrylonitrile copolymer and a styrene/maleic acid copolymer.
The present inventors have revealed that the friction coefficient of a latent-image bearing member increases as temperature rises, depending on temperature characteristics of the binder that occupies most part of the surface of the OPC photosensitive member.
When the friction coefficient of the latent-image bearing member increases as stated above, the friction between the cleaning blade as a cleaning means and the surface of the latent-image bearing member increases. In particular, the friction coefficient abruptly increases when the temperature of the latent-image bearing member becomes higher than 45.degree. C., often causing the vibration of the cleaning blade, a break of the edge of the cleaning blade, or a turnover of the blade which is provided opposite to the direction of movement of the latent-image bearing member, resulting in an extreme lowering of cleaning action.
In order to solve such a problem, the external addition of a lubricant such as a fatty acid metal salt (e.g., zinc stearate) or fine particles of a fluorine compound has been attempted to form a thin layer of the lubricant on the surface of the latent-image bearing member during development so that the friction coefficient on the surface of the latent-image bearing member decreases.
When, however, such a lubricant is externally added to so-called two-component developer, which is a mixture of toner particles and magnetic particles (a carrier), the lubricant may adhere to carrier surfaces (carrier contamination) after long-time use to bring about unstable triboelectric charging of the toner, causing problems such as ground fog, decrease in image density and in-machine contamination due to toner scatter. For a full color image forming apparatus, in particular, the ground fog is transferred in plural times resulting in severe ground fog phenomenon.
As a substitutive means, it has been proposed to provide lubricant coating means in front and the rear of the cleaning means to decrease the friction coefficient of the surface of the latent-image bearing member. This, however, is not preferable since the apparatus becomes large-sized and complicated.
Meanwhile, in recent years, as the image forming apparatus such as electrophotographic color copying machines has become very popular, they are used in various fields and for various purposes and demand for their image quality has become higher and higher. In copying images such as usual photographs, illustrated catalogues and maps, very fine and faithful reproduction without blur is required even at the minute part.
In recent image forming apparatus such as an electrophotographic color copying machine employing digital image signals, a latent image is formed from dots of a given potential, and solid areas, halftone areas and line areas are expressed by changing the dot density. There, however, is a problem that appropriate tone of a toner image can not be obtained based on the dot density ratio of solid black area and the solid white area of the digital latent image when the toner particles are not accurately laid on the dot area and the toner particles extend beyond the dots. Moreover, when the dot size is made smaller to improve resolution in order to improve image quality, it becomes more difficult to reproduce latent images formed from minute dots, tending to cause unsatisfactory sharpness, a poor resolution and, in particular, a poor gradation at the highlight area.
Sometimes the image quality, though good at the initial stage, deteriorates as copying or printing is continued. This phenomenon is attributed to the toner particles of poor developability accumulated in the developing assembly during copying or printing while the toner particles readily participating in development are preferentially consumed.
For the purpose of improving image quality, some developers have been hitherto proposed. Japanese Laid-Open Patent Application No. 51-3244 discloses a non-magnetic toner whose particle size distribution is controlled to improve the image quality. In this toner, most particles have a diameter of 8 to 12 .mu.m, which is relatively coarse. As a result of studies made by the present inventors, such particle diameters make it difficult for the toner particles to be uniformly and densely "laid" on the latent image. In addition, in view of its feature that particles of 5 .mu.m or smaller in diameter are not more than 30% by number and those of 20 .mu.m or larger in diameter are not more than 5% by number, the broad particle size distribution tends to lower the uniformity. In order to form sharp images using such a toner comprised of rather coarse toner particles with a broad particle size distribution, it is necessary to lay toner particles thick to fill the spaces between toner particles so that apparent image density can be increased. Thus, there is a problem of increased toner consumption to achieve given image density.
Japanese Laid-Open Patent Application No. 54-72054 discloses a non-magnetic toner having a narrower distribution than the foregoing, in which, however, the size of particles with a medium weight is as coarse as 8.5 to 11.0 .mu.m. There is room for further improvement as a high-resolution color toner capable of faithfully develop minute dot latent images.
Japanese Laid-Open Patent Application No. 58-129437 discloses a non-magnetic toner having an average particle diameter of 6 to 10 .mu.m, and 5 to 8 .mu.m for the most particles, in which, however, particles of 5 .mu.m or smaller in diameter are as less as 15% by number. This tends to result in image formation lacking sharpness.
As a result of studies made by the present inventors, it has been discovered that the particles of 5 .mu.m or smaller in diameter participate in clear reproduction of minute dots of the latent image and have a main function for the toner to be densely laid on the whole latent image. In particular, in electrostatic latent images on a photosensitive member, edges that contour an image have a higher electromagnetic intensity than the inner area because of the concentration of lines of electric force at the edge, so that the sharpness in image quality depends on the quality of the toner particles gathering at this part. Studies made by the present inventors have revealed that the amount of the particles of 5 .mu.m or smaller in diameter is important in solving the problem on highlight gradation.
However, the particles of 5 .mu.m or smaller in diameter show particularly strong adhesion to the surface of the latent-image bearing member so that the cleaning becomes difficult.
In addition, continuous printing may cause the fast adhesion of the toner and low electrical resistance materials, e.g., paper dust or ozone addition products, to the photosensitive member. In order to scrape off the matter of a low electrical resistance or the toner having stuck thereto, Japanese Laid-Open Patent Application No. 60-32060 or No. 60-136752 discloses the addition of as an abrasive, that is an inorganic fine powder having a BET surface specific area, as measured by the BET method using nitrogen absorption, of 0.5 to 30 m.sup.2 /g.
Although this method is effective in preventing the phenomenon of toner sticking, it is unsatisfactory to achieve the steady cleaning, when applied to a high-resistance color toner having smaller particle diameter as used in the present invention unless charge stability is improved.